Free Volumes, Glass Transitions, and Cross-Links in Zinc Oxide/Waterborne Polyurethane Nanocomposites

The free-volume properties in a system of zinc oxide (ZnO) nanoparticles (20 nm) dispersed in waterborne polyurethane (WBPU) were measured using positron annihilation lifetime spectroscopy. Two glass-transition temperatures (T-g), lower T-g similar to 220 K and higher T-g similar to 380 K of the ZnO/WBPU nanocomposites, were found and both increase with increasing zinc oxide content from 0% to 5%. These two glass transitions are interpreted from two segmental domains of WBPU; the lower T-g is due to soft aliphatic chains and high T-g is due to polar hard microdomains, respectively. The increase in T-g with the addition of ZnO fillers is mainly attributed to interfacial interactions through hydrogen bonding, van der Walls forces, and electrostatic forces between the polymer matrix and zinc oxide nanoparticles. These results are supported by the data from the dynamic mechanical thermal analysis (DMTA). The relationship between the free volume obtained from nanoscopic positron method and the physical cross-link density from macroscopic DMTA method as a result of microphase separation of hard and soft segments in polyurethane is found to follow an exponential function. Chemical properties and surface morphology of nanocomposites were examined by Fourier transform infrared spectroscopy (FTIR) and by atomic force microscopy (AFM).